Since a plastic lens is lighter, hardly broken and capable of being dyed, compared to an inorganic lens, the plastic lens has recently become widely used rapidly in an optical element such as a spectacle lens or a camera lens.
Higher performance has been demanded for a resin for the plastic lens and high refractive index, high Abbe number, low density, high heat-resistance, or the like has been required. So far various kinds of resin materials for the lens have been developed and used.
Among these, an optical material comprised of a polythiourethane-based resin has high refractive index and high Abbe number, and is excellent in the impact resistance, the dyeability, the processability, or the like. The polythiourethane-based resin can be obtained by reacting a polythiol with a polyiso(thio)cyanate compound, or the like.
In a case of using in the plastic lens, the polythiourethane-based resin is required to cause less coloration, have excellent resin color and be transparence. In a case where the quality of polythiol was deteriorated, there were cases where the quality of the obtained resin was also deteriorated.
Patent documents which relate to a process of producing polythiol include the following patent documents.
In Patent Document 1 or Patent Document 2, a method in which 2-mercaptoethanol reacts with epichlorohydrin, the obtained compound reacts with thiourea to obtain an isothiuronium salt, and next the isothiuronium salt is hydrolyzed to obtain the specific polythiol compound is described.
In Patent Document 3, a process of producing a polythiol compound in which the amount of the specific impurities included in 2-mercaptoethanol is set to the predetermined range is described.
In Patent Document 4, a process of producing a polythiol compound in which calcium content included in thiourea is set to the predetermined range is described.